U.S. patent number 5,307,817 [Application Number 07/598,685] was granted by the patent office on 1994-05-03 for biotelemetry method for the transmission of bioelectric potential defferences, and a device for the transmission of ecg signals.
This patent grant is currently assigned to MEDESE AG. Invention is credited to Fritz Grogg, Walter Guggenbuhl.
United States Patent |
5,307,817 |
Guggenbuhl , et al. |
May 3, 1994 |
Biotelemetry method for the transmission of bioelectric potential
defferences, and a device for the transmission of ECG signals
Abstract
To obtain optical one-way transmission of bioelectrical
potential differences between electrodes placed on a patient and
the evaluation apparatus, one of the electrodes placed on the
patient is designated as the reference electrode. The potentials of
all the other electrodes are referred to this electrode, and the
signals derived from the potential differences so obtained are
amplified, multiplexed, converted analog/digital and transmitted in
coded form. At least one of the potential differences recorded by
the evaluation apparatus is derived from the difference between two
transmitted signals. The number of signals transmitted can thus be
less than the number of potential differences recorded. This
process is particularly useful for transmitting electrocardiogram
signals (ECG signals) to an ECG apparatus which produces twelve
standard derivatives. The optical transmission can also be
performed by an optical fibre. The device comprises an emitting
part, preferably provided with several light-emitting elements,
which forms a self-contained apparatus to be carried by the patient
and a receiving part, preferably provided with several
light-receiving elements arranged at fixed places in the
surroundings.
Inventors: |
Guggenbuhl; Walter (Stafa,
CH), Grogg; Fritz (Zurich, CH) |
Assignee: |
MEDESE AG (Zurich,
CH)
|
Family
ID: |
4182839 |
Appl.
No.: |
07/598,685 |
Filed: |
October 25, 1990 |
PCT
Filed: |
January 23, 1990 |
PCT No.: |
PCT/CH90/00014 |
371
Date: |
October 25, 1990 |
102(e)
Date: |
October 25, 1990 |
PCT
Pub. No.: |
WO90/08501 |
PCT
Pub. Date: |
August 09, 1990 |
Foreign Application Priority Data
Current U.S.
Class: |
600/508; 600/523;
128/908 |
Current CPC
Class: |
A61B
5/0006 (20130101); A61B 5/0017 (20130101); Y10S
128/908 (20130101) |
Current International
Class: |
A61B
5/00 (20060101); A61B 005/0402 () |
Field of
Search: |
;128/696,710,908,903,904,699 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Howell; Kyle L.
Assistant Examiner: Schaetzle; Kennedy J.
Attorney, Agent or Firm: Egli International
Claims
We claim:
1. A method for the transmission of bioelectric potential
differences, such as electrocardiogram signals, occurring between
more than two electrodes applied on a patient, from the patient to
an ECG device supplying twelve standard derivations, in the course
of which at least ten ECG electrodes are applied on the patient in
the customary way for obtaining the standard deviations, two of
said at least ten ECG electrodes are defined as first and second
ECG reference electrodes and the remaining ECG electrodes as ECG
signal electrodes, the transfer of potential differences between
the patient and the ECG device being effectuated as an
electromagnetic one-way connection between the electrodes and the
ECG device, said method comprising the steps of:
calculating the difference between an ECG signal obtained from each
ECG signal electrode and an ECG signal obtained at the first ECG
reference electrode yielding signals that are essentially free from
any component corresponding to a common direct voltage content and
also free from any component corresponding to a hum,
determining a reference potential at the patients' end from the
second ECG reference electrode,
transmitting the signals in a one-way serial manner, said step of
transmitting the signals comprises the steps of:
multiplexing the signals to the transmitted,
converting the multiplex signal from an analog to digital
signal,
encoding the digital signal using Manchester encoding or frequency
shift keying-modulation,
modulating a light beam with the encoded signal
transmitting the modulated light beam to a receiving unit,
demodulating the modulated light beam to obtain the encoded
signal,
converting the digital encoded signal into an analog encoded
signal, and
demultiplexing the analog encoded signal to obtain transmitted ECG
signals; and
inputting the transmitted ECG signals to the corresponding inputs
of the ECG device, said reference potential is supplied to the
remaining inputs of the ECG device.
2. The method according to claim 1, wherein the transmission is
performed optically and via an optical waveguide.
3. The method according to claim 1, further comprising the step of
adding additional information to the encoded signal for error
protection in transmission.
4. The method according to claim 3, further comprising the step of
correcting the decoded signal by removing the additional
information added to the encoded signal.
5. A device for the transmission of bioelectric potential
differences occurring between more than two electrodes applied on a
patient to an ECG device in an ECG system comprising at least ten
ECG electrodes applied on a patient and an ECG device for twelve
standard derivations, while two specific ECG electrodes are defined
as first and second ECG reference electrodes and the remaining ECG
electrodes as ECG signal electrodes, having a transmitting unit, a
receiving unit, and at least one optical link in between, the
transmitting unit including:
input protection circuits to which one assigned ECG electrode each
can be connected,
means for computing the difference between signals from respective
input protection circuits, one of said input protection circuits
being assigned to an ECG signal electrode and a second one of said
input protection circuits being assigned to the first ECG reference
electrode so that each signal difference is assigned to an ECG
signal electrode, the output of one of said input protection
circuits assigned to the second ECG reference electrode is
connected with a common ground lead to the means for computing,
a multiplexer for multiplexing the obtained signal differences,
at least one modulatable optotransmitter having a modulation input
through which the output of the multiplexer is linked via the
serial connection of an analog-to-digital converter, and an
encoder, and the receiving unit including:
at least one optoreceiver having an output for the output of the
modulation of a received light beam,
a demultiplexer having an input linked to the output of said at
least one optoreceiver, said demultiplexer is connected in series
to a decoder and a digital-to-analog converter, the outputs of the
digital-to-analog converter, each of which the assigned to the
signal difference formed in the transmitting unit, are connected to
a corresponding input of the ECG device.
6. The device according to claim 5, further comprising:
a first circuit means for adding additional information to an
output of said encoder for error protection in transmission;
and
correction circuit means for removing the additional information
added to the output of said encoder from the output of said
demultiplexer.
7. The device according to claim 5, wherein the transmitting unit
is designed to be carried by the patient as an independent portable
device comprising at least one opto-transmitter arranged on a side
of the transmitting unit which faces away from the patient when the
transmitting unit is carried by the patient, and at least one
opto-receiver arranged at a fixed location within a room
surrounding the patient.
8. The device according to claim 5, wherein the transmitting unit
is designed to be carried by the patient as an independent,
two-part, portable device comprising a portable operating unit, a
portable opto-transmitter unit, the opto-transmitter unit
comprising at least one opto-transmitter arranged on a side of the
opto-transmitter unit which is facing away from the patient when
the opto-transmitter unit is carried by the patient, and an
opto-receiver arranged at a fixed location within a room
surrounding the patient.
Description
The invention relates to a biotelemetry method for the transmission
of bioelectric potential differences, occurring between more than
two electrodes applied on a patient, from the patient to an
evaluator that is intended and designed for processing and
recording bioelectric potential differences, the transmission of
the signals between the patient and the evaluator being effectuated
as electromagnetic and particularly optical one-way connection from
the patient to the evaluator, while there is no electric connection
between the electrodes and the evaluation device, in accordance
with the preamble of claim 1, as well as a device for performing
this method in an ECG system.
An electrocardiogram (ECG) is the recording of the variation in
time of heart action tensions in a living being, in general in a
human patient. For further definitions see DIN 13401 (January
1953).
The analysis of the ECG supplies the physician with important data
on the heart function of the patient. The ECG signals are picked up
at ECG electrodes applied on the patient and arranged at the
patient in the usual way for obtaining the so-called standard
derivations ("left arm, right arm, left foot, right foot, chest
position 1 to 6").
In this process, up to ten cables are connected to the patient,
which connect the ECG electrodes with the ECG device, where the ECG
signals are processed and recorded. These cables constitute a
hindrance to the patient and render the execution of an exercise
ECG, for instance if the response of the patient's heart to
physical exercise is checked on an ergometer, more difficult: the
patient has only very limited possibilities of movement, not least
because the ECG electrodes fall off if due to the weight and the
stiffness of the cables they are pulled on, or if the patient is
moving about violently. Also the psychological strain on the
patient, who is linked to a mains operated device via electric
leads and who reacts with fear of an electric shock, is not to be
disregarded. The effect of this psychological strain can combine
itself with the effect of the real physical strain, which
subsequently falsifies the examination of the heart function.
For this reason, there exists a need for non-electric and, if
necessary, for an immaterial ("wireless") connection between the
ECG electrodes applied on the patient and the ECG device: this
non-electric connection is to easily replace the usual connection
by means of electric leads. In other words, there is a need for a
non-electric transmission method for ECG signals adapted to the
circumstances of the ECG recording from the ECG electrodes applied
on a patient to the ECG device that processes and records the ECG
signals.
There is also a need for a device to whose inputs the ECG
electrodes applied on the patient are connected in exactly such a
way as though they were connected to the ECG device, and whose
outputs are connected to the inputs of the ECG device in exactly
such a way as though they were the ECG electrodes applied on the
patient. By way of this device, customarily designed and arranged
ECG electrodes are to be connected with a customary ECG device, the
connection having to be non-electric and, if necessary,
immaterial.
Under these conditions, it is obvious to devise a connection
between the ECG electrodes and the ECG device by means of
electromagnetic waves. Such a connection requires a transmitting
unit, a transmission link, and a receiving unit. The transmitting
unit needs to be carried by the patient, otherwise the patient
would be linked with the transmitting unit via electrode cables,
which must be rejected as inconsistent and inappropriate. However,
evidently the hindrance for the patient to be eliminated is not
neutralized, until the transmitting part can be carried easily by
the patient. In addition, the transmitting unit must meet the usual
requirements in regard to safety of the patient and defibrillation
stability (defibrillation is a measure against cardiac flutter or
fibrillation). Besides, the considered connection must be able to
transmit a great data flow.
In general, telemetry systems for biosignals are already known.
From the article by N. Kudo, K. Shimizu and G. Matsumoto "Optical
biotelemetry using indirect light transmission" on pages 55-58 of
"Biotelemetry IX" (1987, published by H. P. Kimmich and M. R.
Neuman) the transmission of bioelectric signals from electrodes
applied on a patient to an evaluator is known, in particular, to an
ECG device that processes and records the signals. In this process,
the transmission of the signals is performed optically, if
necessary, in the infrared range, and as one-way link from the
electrodes to the evaluator, and no electric connection between the
electrodes and the evaluator is established.
It is a disadvantage of this known transmission method that the
measuring values are transmitted as analog values: the analog
measuring values obtained at the patient are modulated as pulse
intervals, converted, multiplexed, transmitted, demultiplexed and
demodulated, so that in the end analog values are available again.
Such a procedure does not permit the application of modern
techniques for the reduction of the liability to disturbances of
the transmission and, if necessary, for the correction of
transmission errors, which are known in the field of digital data
transmission.
This known transmission method is notably disturbed, in particular,
by the light of AC light sources, i.e. by the 50 Hz-hum and the 100
Hz-noise of fluorescent tubes, and also by their harmonic
oscillations.
Besides, another disadvantage of this known transmission method is
that the transmitted analog values are by no means adapted to the
circumstances of the ECG recording with an ECG device that supplies
the twelve standard derivations, causing that the known optical
transmission system cannot be used easily instead of the customary
connection by means of electric leads. In particular, the
transmitted values are not fed to the ECG device in an isochronic
manner, because the sampling required for the multiplexing supplies
mutually time-displaced signals on the various channels. In the
case of the known transmission method, the necessary correction of
this time displacement is not provided.
It is a further disadvantage of this known transmission method that
for each of the potential differences between specifically selected
electrodes, which are of interest to the physician, a channel of
the telemetry system is needed so that the number of recorded
potential differences equals the number of transmission channels or
of signals transmitted. In the system described in the quoted
article, only three channels for signals corresponding to potential
differences are provided (the fourth channel is assigned to a
temperature-related signal). When using this art in systems
intended for recording a greater number of potential differences,
such as in a ECG system with twelve standard derivations, twelve
transmission channels would be required.
Besides, there exists a need for a method and a device for
biotelemetry that make it possible to offer not only ten
transmission channels as for electrocardiography with twelve
standard derivations but twenty-two to twenty-four channels as
needed for electroencephalography.
Therefore, it is the object of this invention to remedy the
disadvantages of the known transmission method in a biotelemetry
method of the kind mentioned above, and in particular in a
transmission method via a great number of channels, for example
with eight channels for the transmission of electrocardiogram
signals (ECG signals) to an ECG device supplying twelve standard
derivations, or with twenty-two to twenty-four channels for the
transmission of electroencephalography signals (EEG signals), and
in particular to reduce the required number of transmission
channels.
According to the invention, this object is accomplished in that at
the patient's end one of the electrodes is designated as reference
electrode to which the potentials of all the remaining electrodes
are referred, the signals that were formed by the corresponding
potential difference between the remaining electrodes and the
reference electrode, are amplified, multiplexed, A/D-converted and
transmitted in an encoded way, and at least one of the potential
differences to be recorded in the evaluator was obtained from the
difference between two transmitted signals.
For the forming of potential differences at the patient's end,
preferably all remaining electrodes are paired with the reference
electrode (in the case of four electrodes in a way analog to a
three-phase star connection), while at the end of the evaluator the
potential differences to be recorded are for the one part obtained
from the difference between one pair of transmitted signals each
(in the case of three potential differences in a way analog to a
three-phase delta connection) and for the other part directly from
one transmitted signal each (in the case of three potential
differences in a way analog to a three-phase star connection
against ground potential) so that the number of transmitted signals
is smaller than the number of recorded potential differences.
Preferably, the signals converted from analog to digital are
encoded in the Manchester code or as frequency shift keying and
preferably equipped with additional information for error
protection.
Preferably, for the transmission of electrocardiogram signals (ECG
signals) to an ECG device supplying twelve standard derivations, in
the course of which at least ten ECG electrodes are applied on the
patient in the customary way for obtaining the standard derivations
("left arm, right arm, left foot, right foot, chest position 1 to
6"), the method according to the invention is characterized in that
two specific ECG electrodes ("left foot, right foot") are defined
as first ("left foot") and second ("right foot") ECG reference
electrodes and the remaining ECG electrodes ("left arm, right arm,
chest position 1 to 611) as ECG signal electrodes, for each ECG
signal electrode the difference between the ECG signal obtained
from it and the ECG signal obtained at the first ECG reference
electrode ("left foot") is formed and thus a signal is produced
that is freed from the common direct voltage content and from the
hum ("common mode rejection"), the second ECG reference electrode
("right foot") is used for the determination of a reference
potential at the patient's end, the signals to be transmitted are
transmitted in a one-way serial manner by multiplexing them,
converting the multiplex signal from analog to digital, and
subsequently into an encoded signal to which, if necessary,
additional information for error protection is added, with the
encoded signal a light beam is modulated, the light beam is sent,
transmitted and received, the received light beam is demodulated,
the obtained modulation decoded, if necessary, the decoded signal
is corrected by means of the additional information for error
protection, converted from digital to analog, and demultiplexed in
order to obtain transmitted ECG signals, and the transmitted ECG
signals are fed to the corresponding inputs ("left arm, right arm,
chest position 1 to 6") of the ECG device ("right foot, left
foot"), while the reference potential of the transmitted ECG
signals is supplied to the remaining inputs of the ECG device
("right foot, left foot").
Preferably, the transmission is performed optically via an optical
cable.
A device for performing the method of the invention in its
embodiment designed for the transmission of electrocardiogram
signals (ECG signals), said ECG system comprising at least ten ECG
electrodes applied on a patient ("left arm, right arm, left foot,
right foot, chest position 1 to 6") and an ECG device for twelve
standard derivations, while two specific ECG electrodes ("left
foot, right foot") are defined as first ("left foot") and second
("right foot") ECG reference electrodes and the remaining ECG
electrodes as ECG signal electrodes, is characterized by a
transmitting unit, a receiving unit, and at least one optical link
in between, the sending unit including:
input protection circuits known per se in ECG systems to which one
assigned ECG electrode each can be connected,
computing circuits known per se in ECG systems for obtaining the
difference between signals from two input protection circuits, one
of which is assigned to an ECG signal electrode ("left arm, right
arm, chest position 1 to 6") and the other to the first ECG
reference electrode ("left foot") so that also each signal
difference is assigned to an ECG signal electrode,
a connection of the output of the input protection circuit that is
assigned to the second ECG reference electrode ("right foot"), with
a common ground lead for the computing circuits,
a multiplexer for the obtained signal differences,
at least one modulatable optotransmitter at whose modulation input
the output of the multiplexer is linked via the connection in
series of an analog-to-digital converter, an encoder, and, if
necessary, a circuit for adding additional information for error
protection, and the receiving unit including:
at least one optoreceiver equipped with an output for the output of
the modulation of a received light beam,
a demultiplexer with an input linked to the output of the
optoreceiver(s), if necessary via a connection for correction by
means of the additional information for error protection, and then
via the connection in series of a decoder and a digital-to-analog
converter, and with outputs, each of which is assigned to the
signal differences formed in the transmitting unit and carrying a
signal that can be fed to a corresponding input ("left arm, right
arm, chest position 1 to 6") of the ECG device.
In a preferred embodiment of the device according to the invention,
the transmitting unit is designed as independent device that may be
carried by the patient. Preferably, at least one and preferably
several optotransmitters are arranged on at least one side of the
transmitting unit not facing the patient, and at least one and
preferentially several optoreceivers are arranged at one or several
fixed locations of a room surrounding the patient.
In another preferred embodiment of the device according to the
invention, the transmitting unit is designed as independent
two-part device with an operating unit and an optotransmitter
portable unit, both units capable of being carried by the patient
and possessing at least one and preferentially several
optotransmitters on at least one side not facing the patient, while
at least one and preferentially several optoreceivers are arranged
in one or several fixed locations of a room surrounding the
patient.
In this invention, there is recognized and used the possibility of
forming the 12 standard derivations, in the ECG technology, for the
one part directly from 8 signals obtained by referring them to a
reference electrode and for the other part by forming of the
difference between two signals in the evaluator. The number of
displayable "derivations" could, according to this method and if it
is of interest to the physician, be increased to up to 36.
In the device according to the invention, this kind of acquisition
and transmission of bioelectric potential differences in connection
with a ECG system is reached by using two reference electrodes at
the transmitting end, one of which ("left foot") is used as
reference electrode in the above way and the other ("right foot")
for the determination of the electric reference potential of the
transmitting unit.
In the method according to the invention, individual "derivations"
are calculated from the difference between one pair each of
transmitted potential values, which requires a higher accuracy of
the transmission channels than with the customary direct
transmission of the potential values corresponding to the
"derivations". For example, differences of two approximately equal
potential values have to be formed, which impairs the relative
accuracy of the result. The required higher accuracy of the
transmission channels is ensured, in accordance with the invention,
by digital transmission with a sufficient number of bits, and could
be accomplished in a customary system, e.g. by means of the analog
transmitting method described in the quoted article, only at the
cost of a high instrumental effort (costs, weight).
In order to reduce the weight of the transmitting unit and the
general effort, the method according to the invention, as well as
the method known from "Biotelemetry IX", operates with a one-way
transmission without receive acknowledgement. In order to attain
the highest possible transmission speed and data integrity with
lowest possible error liability, the transmission of the method
according to the invention is digital and encoded according to an
appropriate encoding method, preferentially in the Manchester code
or as frequency shift keying (=FSK). It is a particular advantage
of these encoding methods that their frequency spectra have no
continuous and no effective low-frequency content so that the 50
Hz-hum and the 100 Hz-noise by fluorescent tubes and their harmonic
oscillations can be suppressed. Besides, Manchester code and
FSK-modulation do not require a synchronization of the receiving
clock pulse and the transmitting clock pulse, because they are
autosynchronizing at the receiving end and thus guarantee data
integrity and low error liability for the receiving end in spite of
one-way transmission without receive acknowledgement.
Besides, the transmission of digital data permits adding to them an
additional information for error protection, e.g. for the cyclic
redundancy check (CRC), for the parity check, or according to the
Hamming code. Circuits for the creation of additional information
for error protection at the transmitting end and for the execution
of the relevant checks at the receiving end are well-known to
persons skilled in the art.
The device according to the invention is intended and designed for
replacing easily the customary connection cables, while the
physician does not have to trained or retrained for using the
device according to the invention. The transmitting unit taps the
ECG electrodes, produces the ECG signals, amplifies them, converts
them to the required shape, and transmits them in a definitely
non-electric and, if necessary, in a wireless or cableless way. The
receiving unit reproduces the ECG signals and attenuates them to
the normal value of real ECG signals picked up directly at the ECG
electrodes so that the outputs of the receiving unit can be
connected directly to the corresponding inputs of a customary ECG
device. The ECG device does not "note" any difference, whether the
patient is connected directly by the customary cables or by the
device according to the invention, and the ECG recording is the
same in both cases.
Besides, the optical transmission preferentially used in the method
according to the invention and in the device according to the
invention brings on the advantage of not being subject to legal
provisions and restrictions in regard to transmitting and receiving
licenses.
The method according to the invention, its modifications, and
embodiment examples of the device according to the invention are
explained hereinafter in further detail by reference to the
drawings.
FIG. 1 is a block circuit diagram of an ECG system equipped with a
device according to the invention,
FIG. 2 is a diagrammatically represented embodiment of a input
protection circuit used in the transmitting unit of the device
according to the invention,
FIG. 3 is a diagrammatically represented embodiment of a circuit
for preamplification, difference forming, and "antialiasing"
filtration superposed to the input protection circuit and used in
the transmitting unit of the device according to the invention,
FIG. 4 is a diagrammatically represented embodiment of an output
circuit used in the receiving unit of the device according to the
invention,
FIG. 5 is a diagrammatically and perspectively represented
embodiment of the housing of a transmitting unit equipped with
optotransmitters and used in the device according to the invention,
and
FIG. 6 is a diagrammatically and perspectively represented
embodiment of a headband equipped with optotransmitters and used in
the device according to the invention.
For the explanation of the invention, several requirements to be
met at the entry of the device according to the invention need to
be discussed first.
At the electric transition between ECG electrodes and the skin of
the patient, the transition from the ionic conduction of the body
to the electron conduction of the ECG electrode connections takes
place. This creates a galvanoelectric direct voltage that may take
on several values because of irregularities of the skin and causes
a galvanoelectric direct voltage to exist also between two ECG
electrodes. It is to be suppressed in signal processing because it
is significantly greater than the useful signal of approx. 1 Mv. If
the galvanoelectric direct voltage is suppressed as usual only
after a preamplification of the ECG signals in the input amplifier,
this occasions that these direct voltages are to be amplified in a
linear way.
In addition, as little current as possible is to flow by way of the
ECG electrodes because it alters the chemical composition of the
skin and provokes polarization voltages which may vary a great deal
with regard to time. The input currents should remain smaller than
0.1 fA and the input amplifier should have a high input impedance
of higher than 2 Mohm.
Due to the patient and his/her environment, aside from the desired
useful signals several superimposed disturbing voltages appear at
the output of the ECG electrodes, the greatest of which, i.e. the
50 Hz-hum and its harmonic oscillations, appear as common-mode
voltage and can be suppressed by means of the appropriate,
principally known techniques: a common mode rejection rate (CMRR)
of 80 dB is desirable.
The frequency response of ECG signals is of a kind that frequencies
above 100 Hz may be neglected, while the ECG signals located within
the frequency band from 0.1 Hz to 100 Hz are to be transmitted and
processed as distortion-free as possible, both with regard to phase
and amplitude.
The inputs of the device according to the invention, and its
sensitive input amplifiers are to be protected from overdrive or
excitation by high-frequency parasitics. On the other hand, the
remaining direct voltage and interference are to be separated from
the useful signal, which can be achieved by means of high-pass and
low-pass filters with different time constants (approx 3 s and
approx 4 Ms). In addition, the inputs of the input amplifiers
should bear or be shielded from the high voltage pulses (3 kV
during 5 ms) occurring during a defibrillation. This can be
achieved in a known manner by way of signal contraction via Zener
diodes connected in an anti-serial way: so-called bidirectional
Zener diodes developed for these specific aims are for sale.
The patient's safety is to be guaranteed in a legally stipulated
manner, e.g. according to the Swiss "SEV-Norm TP62/d", which
provides that in the case of a failure of the device 5 mA at the
most, and during normal operation 100 fA at the most, may flow
through the patient.
For explaining the invention some requirements in regard to the
electromagnetic and, in particular, optical transmission need to be
discussed, too.
The electromagnetic and, in particular, the optical transmission of
the ECG signals could be effectuated in parallel on several
channels or serially on one channel only: considerations as regards
economic efficiency and portability of the transmitting unit lead
to the selection of the serial transmission on one single channel,
thus requiring that the signals need to be multiplexed prior to
transmission and demultiplexed after the transmission. However,
then the frequency band of the signal to be multiplexed must have
an upper limit which according to the Shannon theorem does not
exceed half of the sampling frequency of the multiplexer: this is
achieved by means of a so-called "antialiasing" filter, e.g. a
Butterwork filter of the 2nd order with a limit frequency of 180 Hz
for a sampling frequency of 600 Hz at the multiplexer.
The transmission in series requires multiplexing, however,
customary multiplexers are not able to process the ECG signals in
the millivolt range, and their input impedances do not meet the
requirements of ECG input amplifiers. In addition, direct voltage
contents need to be suppressed in the ECG signals, which--as has
been described hereinabove--is done by using filters whose time
constant of approximately 3s leads to settling times that are
significantly longer than the connecting times of the ECG signal at
the multiplexer which, for this reason, has to be arranged after
such filters.
For these reasons, the device according to the invention possesses
one input amplifier each per ECG signal, and multiplexing is done
after the input amplifiers. The possibility of using
analog-to-digital converters with integrated multiplexers is an
advantage.
In a customary ECG-device, there exists the possibility of tapping
the currently needed signals in a bipolar-differential manner by
means of difference amplifiers, thus suppressing also the
common-mode voltage as against the reference potential ("right
foot"). As a consequence, by using the method according to the
invention and the device according to the invention the suppression
of the common-mode voltage taking place in the ECG device is not
effective in suppressing the common-mode contents of the currently
evaluated ECG signals which are decisive for transmission.
Therefore, the common-mode voltage must, as far this is possible,
take place already prior to the transmission of the ECG signals,
preferentially in the input amplifiers of the device according to
the invention, in order to avoid an overdrive of the transmission
channels and of the battery-powered input amplifiers. For this
reason, their ground lead is connected with the patient in a
high-resistance manner, which is done preferentially at the ECG
electrode applied on the right foot of the patient, which is not
directly used for purposes of examination: hence, this electrode
becomes the second ECG reference electrode ("right foot") and
determines the reference potential at the patient's end. At the
receiving end, the connection for the second ECG reference
electrode ("right foot") is not needed anymore, however, since it
exists its potential must not remain undefined, and for this reason
it is connected, if necessary, in a high-resistance manner with the
reference potential of the receiving unit.
In an ECG device used in a customary way, with ECG electrodes
directly connected to it, all ECG signals are available to the ECG
device in parallel: the ECG device may at all times access all ECG
signals which are referred to the potential of the ECG electrode
located at the right foot of the patient (reference potential).
However, when using the method according to the invention and the
device according to the invention, the ECG signals are transmitted
serially. Since there is no acknowledgement by the receiver to the
transmitter stating which signals are currently needed by the ECG
device at a certain moment, the ECG signals need to be prepared
again in parallel. The simplest solution is to refer all ECG
signals prepared at the receiving end to one single common
reference voltage, i.e. to the reference potential of the receiving
unit. Hence, the connection for the first ECG reference electrode
("left foot") at the receiving end is not needed anymore, however
since it exists, its potential must not remain undefined and it is,
if necessary, connected with the reference potential of the
receiving end in a high-resistance manner.
In existing ECG devices real difference amplifiers are invariably
used for the evaluation of the ECG signals, because they need only
a small number of them, that is one difference amplifier per ECG
signal recorded, for the recording is, if requested, shifted from
one to the other ECG signal.
When using the method according to the invention and the device
according to the invention, one difference amplifier for each
transmitted ECG signal would have to be provided according to this
customary procedure, and at its two inputs one operational
amplifier (buffer) each would have to be connected in series in
order to ensure a very high-resistance input of the amplifier.
Thus, a great number of difference and operational amplifiers would
be required, bringing on problems in regard to cost and weight.
However, since all ECG signals at the transmitting end are
invariably referred to the same reference voltage of the first ECG
electrode ("left foot"), it is not necessary to use real difference
amplifiers. Each ECG signal may be fed to the non-inverting input
of an assigned operational amplifier, because it is a
high-resistance input. For the correct forming of the desired
differences all ECG signals must be equally amplified in their
respective operational amplifiers: this is achieved by adjusting
the amplification at the other inverting input of the respective
operational amplifier. After all, the output of the operational
amplifier, which is assigned to the first ECG reference electrode
("left foot"), is connected with the inverting inputs of the
operational amplifiers, which are assigned to the other ECG
signals.
An advantage of this embodiment is the input amplifier and that
signals are available by means of which a carry-along circuit (a
so-called bootstrap) can significantly increase the effect of the
cable shields. Since for practical reasons it is not possible to
shield every input separately, the shield for all inputs is carried
along on one and the same potential. This potential is determined
in a separate operational amplifier as mean value of selected ECG
signals. Because the summation at this operational amplifier is
inverting, a second operational amplifier for the inversion of the
obtained shield signal is provided.
The requirement that the ECG device should not "note" any
difference, whether the patient is connected directly by the
customary cables or by the device according to the invention,
occasions that the transmitted signals need to be attenuated at the
receiving end in the same proportion as they were amplified at the
transmitting end. In the receiving unit the corresponding interface
between the outputs of the demultiplexer and the inputs of the ECG
device is quite simple: the ECG signals at the outputs of the
demultiplexer are attenuated in a potentiometer-type resistor in
merely such a way that they correspond to the normal values of real
ECG signals that are picked up directly at the ECG electrodes, so
that they can be fed directly into the respective inputs of a
customary ECG device. The noise from the digital-to-analog
conversion is eliminated by the low-pass filters included in the
ECG device anyway, the high-resistance inputs of the ECG eliminate
the problem of the output resistance at the demultiplexer. Besides,
as has been mentioned before, both inputs of the ECG device for the
ECG reference electrodes ("right foot, left foot") are connected to
the reference potential of the receiving unit. Parts of the
receiver of this device decribed hereinabove as accessory of a
customary ECG system can also be integrated into the ECG device,
thus reducing the entire instrumental effort. In general, the
effort for the electrical safety of the patient in the receiving
unit and in the ECG device can be vastly reduced in relation to the
customary embodiment linked galvanically to the patient, because
the electromagnetic and, in particular, optical link causes the
electrical isolation.
In regard to the two transmission methods in free air or via an
optical waveguide, there exist the following possibilities for use
of the invention. Both transmitting and receiving unit can be
switched over to one or another transmission method by actuation of
a changeover switch and thus be used for both transmission
methods.
The transmission in free air is particularly suitable if the
patient should be capable of moving about freely, for example, as
is necessary for ergonometric measurements.
The transmission via an optical waveguide is particularly suitable
for application in an operating theater or an intensive care unit
where the disturbances of the transmission by light sources can be
very heavy. The transmitting unit can be applied on the bed of a
bedridden patient. The optical transmission will then be
effectuated in a very simply way via a disconnectable plug-in
connection of the waveguide at the transmitting unit and at the ECG
device, so that the patient can be "connected and disconnected"
rapidly and without difficulty. The transmission via a waveguide is
also suited for the remote monitoring of patients from a vigilance
station, since the waveguide transmits the signals without
difficulties across distances of up to 50 m, without external
disturing influences and without requiring an amplification.
The invention offers great advantages when used in emergency
medicine. A transmitting unit is applied on a victim of an accident
still at the site of the accident, which incessantly transmits the
ECG signals to the ECG device, at first in the ambulance vehicle,
later on in the operating theater or in the ward.
FIG. 1 is a block diagram of an entire ECG system equipped with a
device according to the invention. In this system,
electrocardiogram signals (ECG signals) are transmitted from ECG
electrodes 2 applied on the patient 1 to an ECG device 3 that
processes the ECG signals and records them, as symbolized in 4. The
ECG electrodes 2 are applied on the patient 1 in predefined areas
that have become customary in medical practice for obtaining the
so-called standard derivations "left arm, right arm, left foot,
right foot, chest position 1 to 6". Two of these ECG electrodes
("left foot, right foot") serve as reference for the acquisition of
the signals of the other electrodes and are hereinafter defined as
first ("left foot") and second ("right foot") ECG reference
electrodes. For reasons of clarity, the remaining ECG electrodes
are defined as ECG signal electrodes.
In a normal customary system not using the invention, the branch 5
of output leads of the ECG electrodes 2 and the branch 6 of input
leads of the ECG device 3 constitute one and the same branch: the
inputs of the ECG device 3 are assigned to the ECG electrodes 2,
accordingly, and designated in the corresponding way ("left arm,
right arm, chest position 1 to 6"). However, if the invention is
used the device according to the invention is connected between the
branches 5 and 6.
The device according to the invention includes a transmitting unit
7, implied in the upper part of FIG. 1 by a curved parenthesis, and
a receiving unit 8, implied in the lower part of FIG. 1 by a curved
parenthesis. Between the transmitting unit 7 and the receiving unit
8 there is at least one optical link 9, implied in the central part
of FIG. 1 by a curved parenthesis and explained hereinafter in
further detail.
The transmitting unit 7 includes at least one modulatable
optotransmitter, in the embodiment according to FIG. 1 it is two
optotransmitters 10 and 11 which can be activated alternately or
simultaneously. The optotransmitter 10 may consist of one or
several transmitting elements and sends modulated light 15,
preferentially infra-red light, into the surrounding free room. To
the optotransmitter 11 a flexible optical waveguide cable 12 can be
connected by means of a (not represented) disconnectable plug-in
connection. The optotransmitter 11 will then feed light,
preferentially infra-red light, into the connected optical
waveguide cable 12. Both optotransmitters 10 and 11 are
electro-optical transducers capable of converting an electric
signal into modulated light and preferentially into infra-red
light.
The receiving unit 8 includes at least one optoreceiver, in the
embodiment according to FIG. 1 it is two optoreceivers 13 and 14
which can be activated alternately or simultaneously. The
optoreceiver 13 may consist of one or several receiving elements
and receives light 15, preferentially infra-red light, from the
surrounding free room. To the optoreceiver 14 the flexible optical
waveguide cable 12 can be connected by means of a (not represented)
disconnectable plug-in connection. The optoreceiver 14 will then
receive the light, preferentially infra-red light, supplied by the
connected optical waveguide cable 12. Both optoreceivers 13 and 14
are electrooptical transducers capable of converting the modulation
of light and preferentially of infra-red light back into a
demodulated electric signal.
Thus, the optical link 9 includes alternately the free air between
the optotransmitter 10 and the optoreceiver 13, or an optical
waveguide designed as flexible optical waveguide cable 12. It must
be understood that other embodiments of the optical waveguide, for
example as single optical fibres with or without disconnectable
plug-in connections are possible and can be used. It is
recognizable that the transmission of signals from the transmitting
unit 7 to the receiving unit 8, i.e. the transmission of ECG
signals from the ECG electrodes 2 to the ECG device 3 is optical in
any case, i.e. non-electrical, and alternately immaterial, i.e
wireless and cable-less. Between the ECG electrodes 2 applied on
the patient 1 and the ECG device 3 no electric connection is
established.
In the transmitting unit 7 an input amplifier 20 is provided which
hereinafter is explained in further detail in the context of FIGS.
2 and 3. The ECG signals picked off by the ECG electrodes 2 are fed
to the input amplifier 20 by way of the branch 5. The input
amplifier 20 possesses an assigned input channel for each ECG
electrode 2, to which one assigned ECG electrode each may be
connected, and guarantees the required input protection on these
input channels with the help of input protection circuits that are
principally known in ECG systems.
FIG. 2 is a diagrammatic representation of an embodiment of such a
circuit for input protection as provided at each input channel,
right at the input of the input amplifier, i.e. practically at the
socket-contacts which receive the contactors of the leads of the
branch 5 on the housing of the input amplifier 5. The lead 40 is
connected with a lead of the branch 5. The RC element consisting of
the resistor 41 and the capacitor 42 with a limit frequency of
approximately 3 kHz permits that the useful signal contents
penetrate while it dampens the high-frequency contents. For
shielding against defibrillation impulses voltage limitation by
means of the serially connected pair of Zener diodes 43 is
provided, which limits the voltages that arrive at the amplifier
inputs and protects the subsequent electronic circuits from being
destroyed. The lead protected in such a manner leads to further
signal processing in the input amplifier 20, which is now explained
in further detail.
Following the input protection, the input amplifier 20 forms for
each signal electrode the difference between ECG signal picked up
from it and the ECG signal picked up from the first ECG reference
electrode ("left foot") (cf. for more information see description
herinafter in connection with FIG. 3). As a consequence, each
signal difference formed in this way is assigned to an ECG signal
electrode. The output of the input protection circuit assigned to
the second ECG reference electrode ("right foot") is connected with
a common ground lead of the computing circuits in order to adapt
the reference potential of the computing circuits, and of the input
amplifier 20 and the transmitting unit 7 in general, to the
reference potential at the patient's end.
FIG. 3 is a diagrammatic representation of an appropriate circuit
for preamplification and difference forming which--as has been
mentioned hereinabove--is located after the input protection
circuit. It is always by way of the assigned input projection
circuit (e.g. such as described in connection with FIG. 2) that the
lead 50 is connected with the second ECG reference electrode
("right foot") and the lead 51 is connected with the first ECG
reference electrode ("left foot"). The leads 52 and 53 are
connected to one signal electrode each by way of the corresponding
assigned input protection circuit and represent in FIG. 3 all
remaining, not represented ECG signal electrodes, which is also
implied by the arrow 54. Each ECG signal is fed to the
non-inverting input of an assigned operational amplifier 55, 56,
57, etc., since it is a high-resistance input. For the appropriate
forming of the desired differences all ECG signals must be
amplified in its corresponding operational amplifier 55, 56, 57,
etc. by the same and correct factor: on the one hand, this is
reached by means of the feedback resistors at the operational
amplifiers 55, 56, 57, etc. and on the other hand by adjusting the
amplification with the variable resistor in the feedback loop at
the operational amplifier 55. The output of the operational
amplifier 55, which is assigned to the first ECG reference
electrode ("left foot"), is connected by way of a resistor of the
corresponding feedback chain with the inverting inputs of the
operational amplifiers 56, 57, etc., which are assigned to the ECG
signal electrodes (the arrow 54 symbolizes the corresponding
connection with the not represented operational amplifiers).
The output of each operational amplifier 55, 56, 57 is also
connected via one resistor each with the inverting input of the
operational amplifier 58 so that the output of this operational
amplifier 58 carries a signal representing the mean value of the
ECG signals selected in such a way. Because the summation at this
operational amplifier 58 is inverting, another operational
amplifier 59 is provided for the inversion of the received signal
which is fed to the cable shields 60, 61, 62, etc., in order to
carry on their potential ("bootstrapping"). The operational
amplifiers 63, 64, etc. connected as non-inverting,
voltage-controlled voltage sources and serve the purpose of buffer
amplification and impedance conversion. Their output leads to the
operational amplifiers 65, 66, etc. connected for purposes of
"antialiasing" filtration as Butterworth filters of the 2nd order.
These "antialiasing" filters supply the ECG signals to be processed
further on the output leads 67, 68, etc. with reference to the
ground lead 69.
It is by way of one "antialiasing" filter each that is summarized
in a diagrammatic way in the block 21 in FIG. 1 and whose function
has been described hereinabove that the signal differences from the
input amplifier 20 are fed to a multiplexer 22 which in turn passes
them on to an analog-to-digital converter 23. The digital values
obtained in the analog-to-digital converter 23 are encoded in an
encoder 24 in the Manchester code or as FSK modulation. The purpose
and the advantages of this way of encoding have already been set
forth hereinabove. If necessary, an additional information for
error protection is added in this place with the help of an
appropriate circuit, e.g. for the cyclic redundancy check=CRC, or
for the parity check according to the Hamming code.
From the output of the encoder 24 the encoded data is supplied to a
modulation input of an optical transmitting unit 25 including the
optotransmitters 10 and 11 and their feeder and control circuits.
Thus, the modulation signal electrooptically transduced by the
optotransmitters 10 and 11 corresponds to the signal differences
formed in the input amplifier 20, while this modulation signal is
to a large degree freed from the direct voltage content and the hum
of the ECG signals as a consequence of the previous signal
processing ("common mode rejection").
In the receiving unit an optical receiving unit 26 is provided
including the optotransmitters 13 and 14 and their feeder and
output circuits (transimpedance conversion of small currents to
processable voltages). Thus, the signal electrooptically transduced
by the optoreceivers 13 and 14 is demodulated in the optical
receiving unit 26 and the modulation signal obtained is prepared
for further use in a circuit 27: according to the frequency
spectrum of the Manchester code or the FSK modulation, the
continuous and low-frequency contents of the modulation signal
(including the 50 Hz-hum and the 100 Hz noise by fluorescent tubes
and their harmonic oscillations), and the high-frequency noise and
disturbance contents are eliminated by means of filters; at the
same time an amplification to the required signal value for further
use takes place. Such a signal processing is principally known and
does not need to be described in further detail.
The modulation signal, processed in the circuit 27 and available at
one of its modulation outputs, is supplied in a decoder 28 which
restores the digital values processed in the encoder 24 in the
Manchester code or as FSK modulation. If necessary, a correction is
made in this place with the help of an appropriate circuit and with
the help of the additional information for error protection added
in the transmitting unit.
The digital values obtained in this way are fed to a
digital-to-analog converter 29 in order to regain the analog
signals. During the simultaneous smoothing of the analog signals
(low-pass filtration) an interpolation of the support values takes
place, which causes the correction of the time displacement due to
multiplexing. Hence, the analog signals are made isochronic again
and by this it becomes possible to supply them isochronically to
the ECG device.
The analog signals obtained and processed in this way are supplied
to a demultiplexer 30. It is equipped with outputs each of which is
assigned to a signal difference formed in the input amplifier 20 of
the transmitting unit 7. The signals from these outputs of the
demultiplexer 30 are still attenuated in a circuit 31 to values
that correspond to the normal values of the real ECG signals picked
off directly at the ECG electrodes 2, thus permitting to feed them
directly to the corresponding inputs of the ECG device 3 by way of
the branch 6. However, since only the ECG signals from the ECG
signal electrodes are transmitted via the optical link 9 and
supplied to the corresponding inputs of the ECG device ("left arm,
right arm, chest position 1 to 6") by way of the branch 6, the
potential at the remaining inputs of the ECG device ("right foot,
left foot") does not make any useful contribution; for this reason,
these two last inputs are connected to the reference potential of
the ECG device (in a not represented way).
As can be seen, the transmission of the ECG signals by way of the
optical link 9 is effectuated serially as one-way transmission data
towards ECG device, both the encoder 24 and the decoder 28 being
intended and designed for the processing of the Manchester code or
the frequency shift keying modulation.
FIG. 4 is a diagrammatic representation of an embodiment of an
output circuit used in the receiving unit of the device according
to the invention, which is represented in a symbolic way in the
block 31 of FIG. 1. Since the transmitted and thereupon decoded
signals already correspond to the desired signals, only their value
has to be adjusted in a way that the ECG device does not "note" any
difference, whether the patient is connected directly by the
customary cables or by the device according to the invention. This
requires only a linear attenuation achieved by means of the
represented potentiometer-type resistor with the resistors 80, 81.
The input lead 82 of this circuit is connected with one of the
outputs of the demultiplexer 30 and the output lead 83 is linked
with one of the corresponding inputs of the ECG device ("left arm,
right arm, chest position 1 to 6") by way of a lead of the branch
6.
In a modification, the transmitting unit 7 may be designed as
independent integrated device which the patient can carry, for
example, at the belt, on the back or strapped to him/her in another
fashion.
FIG. 5 is a diagrammatic and perspective representation of a
housing 90 of such a transmitting unit 7, which may be carried by
means of a belt 91. This housing 90 is equipped with one or
several, for example four optotransmitters 92, 93, 94 (the fourth
is not visible in FIG. 5). These optotransmitters are for instance
infra-red transmitters and are located at the sides of the
transmitting unit 7, or of its housing 90, not facing the patient,
for example one each at the upper and at the three non-engaged
lateral walls of the housing 90. At the housing 90, a socket 95 for
the connection of a flexible waveguide cable 12 is provided.
In a modification, the transmitting unit 7 may be designed as
independent two-part device. One part is an operating unit which
the patient may carry at the belt, in the pocket, or strapped to
him/her in another fashion. The other part is an optotransmitter
portable unit which may be carried by the patient as headband.
FIG. 6 is a diagrammatic and perspective representation of such a
headband 100 having herein three optotransmitters 101, 102, 103 on
its side not facing the patient. The optotransmitters 101, 102, 103
are linked with each other and with the operating unit by means of
the thin cable 104. The cable 104 can hardly affect the patient,
for its two ends are connected to parts which the patient is
carrying with him/her. The operating unit has not been represented
because in a possible embodiment it is very similar to the
transmitting unit 7 represented in FIG. 5, for example, its housing
is the same as the housing 90 represented in FIG. 5, just without
the optotransmitters.
For receiving the radiation emitted by these optotransmitters at
least one and preferentially several optoreceivers are arranged in
one or several fixed locations of a room surrounding the patient,
for example in the middle of two or three walls of the room, where
the patient is, or even at the ECG device itself.
The description as set forth hereinabove concerning ten
transmission channels as in electrocardiography with twelve
standard derivations, can easily be adapted to another multiplicity
of channels, for example to the twenty-two to twenty-four channels
needed in electroencephalography.
The description as set forth hereinabove concerning optical
transmission can also be adapted easily to another kind of
electromagnetic transmission of microwaves, as is well-known in the
state of the art of telecommunications.
In general, the embodiment of the invention is by no means limited
to the method as hereinabove set forth as a mere example and the
corresponding device. Many equivalent methods and devices whose
embodiment is within the scope of this invention are known to those
skilled in the art.
* * * * *